Use of photocatalytic preparations of colloidal titanium dioxide for preserving the original appearance of cementitious, stone, or marble products

ABSTRACT

Use of photocatalytic preparations of colloidal titanium dioxide optionally doped with a metal chosen from groups I-VA, and the lanthanide and actinide series of the periodic table, for preserving the original appearance of cementitious, stone, and marble products.

FIELD OF THE INVENTION

The present invention refers to the use of colloidal photocatalyticpreparations of titanium, dioxide (TiO₂) for maintaining the originalappearance of cementitious, stone, or marble products.

PRIOR ART

The conservation of architectural assets depends to a considerableextent on the environment in which they are located. The problem of thepreservation of such assets assumes important dimensions if it isconsidered that the action of atmospheric agents has become increasinglyaggressive over time.

The conservation of architectural structures, buildings and artefactssubject to outdoor exposure has to do with forces that act in animportant way on the surface is of the constructions or in the layersimmediately below the surface and that regard the depositing of organicand inorganic materials which usually adhere to the aforesaid organicsubstrate, and the action of acid rain.

One of the main problems of architectural cementitious, stone, or marbleproducts therefore regards the constant maintenance over time of theiroriginal appearance, which is undermined by the natural process ofageing due to the influence of atmospheric agents.

The need is therefore particularly felt for economical systems ofintervention which enable conservation of the aesthetic characteristicsof the surface of constructions made of cementitious, stone, or marblematerial.

With the aim of protecting such artefacts, various techniques areresorted to, the most common of which consist in applyingwater-repellent products, such as silicone solutions that are stable inalkaline environments and are stable to light and weather.Unfortunately, the progressive increase in the pollutants present in atypical metropolitan environment, perhaps also on account of theirfrequently acidic nature, has markedly limited the duration of this typeof protection, with consequent aesthetic degradation of the artefact.

In order to overcome this new problem, various solutions have beenproposed. For example, the patent IT 1286492 (in the name of the presentapplicant) illustrates a hydraulic binder for cementitious compositionswhich comprises in its mass a titanium-dioxide based photocatalyst thatis able to oxidize, and hence neutralize, the polluting substancespresent in the environment. This type of solution is certainly valid asregards the protection of the surface of the artefact from any possibledeterioration caused, for example, by wind abrasion, but clearly itimplies the use of enormous amounts of photocatalyst as compared tothose actually needed. In fact, the photocatalyst is present not only onthe surface of the artefact or in the immediately underlying layers,i.e., in the areas where its presence proves useful, but in the entirecementitious mass, which, for the most part, will never come intocontact with external agents.

Alternative solutions have also been proposed (see, for example, theEuropean patent, application No. EP 0 885 857 in the name of the presentapplicant) which regard cementitious levelling plasters containingvarious polymeric additives and photocatalysts that are able to oxidizethe polluting substances present in the environment, but thesecementitious-based formulations are white, and cannot be used, forinstance, on marble or stone materials without consequently modifyingtheir aesthetic characteristics.

The use of anatase, which is one of the tetragonal crystalline forms oftitanium dioxide, as photocatalyst for the oxidation of organicpollutants has been known for some time, also in the form of colloidalpreparations.

The patent EP 784034 (in the name of Matsushita Electric Works)describes substrates obtained by depositing titanium dioxide on thesurface of a substrate by deposition of a solution containing ammoniumtitanium fluoride, followed by calcining. The patent EP 614682 (in thename of Fuji Electric) illustrates a titanium-based ortitanium/activated carbon-based photocatalyst fixed on a fluororesin toobtain sheets or panels to be applied externally on buildings forremoving low concentrations of NO_(x).

The incorporation of metal ions (doping agents) in these preparationsalters the photocatalytic activity of titanium dioxide to a substantialextent. The most important parameters are the type of ion, theconcentration of the dopants, and the thermal treatment useful for theformation of the photocatalyst.

Brezová V. et al., J. Photochem. Photobiol. A: Chem., 109, (1997),177-183, analyze the influence of various metal ions and theirconcentrations in a particular application of titanium dioxide asphotocatalyst, namely, in the oxidation of phenol. In particular,preparations of colloidal titanium dioxide are described, in which thetitanium dioxide is doped with various metals at 5% atom/atom, followinga procedure whereby colloidal titanium dioxide is first prepared, andnext the salt of the doping metal is mixed under heating. Among thevarious metal ions tested, cerium is reported to reduce thephotocatalytic activity of the colloidal titanium dioxide. In thepublication, as in other previous publications on this subject, nomention is made of the use of the said photocatalytic products oncementitious materials.

The patent EP 633964 (in the name of Fujisawa, Hashimoto, and IshiharaSangyo Kaisha) describes a TiO₂ based photocatalyst preferably dopedwith V, Fe, Cu. Co, Ni, Zn, Ru, Rh, Pd. Ag, Pt, or Au, and fixed on afluorinated polymer for adhesion to the substrate. This photocatalyst isuseful for purifying air, but also water, from various undesiredsubstances.

SUMMARY OF THE INVENTION

It has' now been surprisingly found that colourless colloidalpreparations of titanium dioxide, or of one of its precursors, possiblydoped with elements chosen from groups I-VA, and the lanthanide andactinide series of the periodic table, or mixtures thereof, preferablymagnesium, cerium, niobium or lanthanium, wh n applied on the surface ofcementitious, stone, or marble products, are able to preserve theoriginal appearance of the surface, without altering the characteristicsof the cementitious, stone, or marble material.

The solution devised thus regards a system for treating cementitious,stone, or marble surfaces by using titanium dioxide-based colourlesscolloidal suspensions, the titanium dioxide being possibly doped withelements chosen from groups I-VA, and the lanthanide and actinide seriesof the periodic table.

DETAILED DESCRIPTION OF THE INVENTION

The present invention refers to the use of colourless colloidalpreparations of titanium dioxide or one of its precursors, for theconservation of the original appearance of cementitious, stone, andmarble products, by means of application of said preparations on thegiven surface.

The colloidal preparations of titanium dioxide or one of its precursorsuseful for the purposes of the present invention may contain a metal ionchosen from groups I-VA, and the lanthanide or actinide series of theperiodic table, and preferably comprising lithium, beryllium, magnesium,scandium, yttrium, lanthanium, cerium, niobium, vanadium, zirconium, andmixtures thereof. More preferably, the colloidal preparations oftitanium dioxide or one of its precursors according to the presentinvention are doped with ions selected from the group consisting ofmagnesium, cerium, niobium, and lanthanium.

The amount of doping agent, i.e., of metal ion present in thepreparations useful for the purposes of the present invention, rangesfrom 0.1 to 5% (percentage expressed as metal-ion atoms with respect tothe atoms of titanium), preferably from 0.1 to 1%.

Preferably, the titanium dioxide present in the preparations accordingto the invention is prevalently in the form of anatase; i.e., theparticles of photocatalyst have an anatase structure at least for 75%.

As precursor of titanium dioxide useful for the purposes of the presentinvention is meant any product that is able to produce titanium dioxideprevalently in the form of anatase, possibly with appropriate types ofthermal treatment. For example, TiCl₄, TiOSO₄, titanium alkoxide (forinstance, titanium isopropoxide) can be effectively used as precursorsfor the purposes of the present invention.

By polluting substances that are oxidized by the photocatalysts of thepresent invention are meant the organic substances that may be presentin the environment as a result of motor-vehicle exhaust or industrialemissions, such as benzene, aromatic volatile compounds, pesticides,phenols, and benzofluorides, without excluding inorganic compounds, suchas nitrogen oxides (NO_(x)) which can be oxidized to yield nitrates.

By cementitious artefact is meant any product in the hardened statederiving from a cementitious composition or cementitious mixture, bycementitious composition or cementitious mixture being meant anycomposition in which the binder is mixed with water and possibly withaggregates of various grain size. The cementitious compositions thusinclude both cementitious pastes, i.e., pastes consisting of binder andwater (devoid of aggregates), and conglomerates, i.e., mixtures ofwater, cement and aggregates.

The aggregates or inert substances may be coarse aggregates, such ascrushed stones or gravel, or fine aggregates, such as sand, and areclassified according to UNI 8520 Standards. Examples of conglomeratesare mortars (mixtures of binder, water, and fine aggregate) andconcretes (mixtures of water, binder, fine aggregate, and coarseaggregate). Among the cementitious products preferred according to thepresent invention are cited the so-called “architectural concretes”,i.e., in situ castings with non-treated surfaces (plane or shaped) ortreated surfaces (for example, sanded).

By marble or stone products are meant the marbles proper, i.e.crystallized metamorphic limestones, “calcetiri”, cipollinos, limestone,dolomites, polishable limestone breccias, onyx marbles, serpentines, andophicalcites; granites proper, i.e., intrusive magmatic rocks consistingof quartz, sodium and potassium feldspars, and micas, other intrusivemagmatic rocks (diorites, gabbros, etc.), and the correspondingporphyritic-structured effusive magmatic rocks, metamorphic rocks, suchas “gnelsa” and “serizi”; travertine and the so-called commercial stone,such as sandstone, tuff, quartzites, mica schists, slates, basalts, etc.Titanium dioxide in colloidal form is prepared using sol-gel techniquesso as to have particles of a size of between 10 and 200 Å, preferablybetween 50 and 100 Å.

In contrast with what is reported in the prior art, hydrolysis of thetitanium dioxide takes: place directly in the presence of the salt ofthe doping element by co-precipitation or mixing. In fact thepreparation of two distinct colloidal suspensions, one containing thetitanium dioxide and the other the salt of the doping metal, which aresubsequently mixed together, is described in the prior art.

The colloidal suspension is then treated so as to obtain a colloidalfilm over the material that is to be preserved, a necessary step forassessing the photocatalytic action of titanium dioxide in the presenceof the various types of pollutants.

In particular, the colloid in aqueous suspension may be sprayed orapplied using a brush directly on the product in small successiveamounts until the desired thickness is reached.

UV and visible-light measurements using diffused-reflectancespectroscopy reveal the minimum quantity suitable for guaranteeingmaximum light absorption by the TiO₂.

As an alternative, the colloidal suspension can be vacuum-dried in orderto obtain a titanium-dioxide powder which can be preserved for anunlimited period and re-suspended in water, maintaining its colloidalproperties.

When applied on the surface of cementitious, stone, or marble products,the colloidal preparations according to the present inventioneffectively protect the product from alterations due to organicpollutants, and in general from atmospheric agents, by causing theirphoto-oxidation.

Generally, when inorganic pollutants do not find an organic substrate toadhere to, they encounter greater difficulties in depositing on thesurface of the product

As compared to the methods of the prior art, the application of thepreparations in question guarantees a form of protection of theabove-mentioned products that is long-lasting by using amounts ofmaterial, namely titanium dioxide or one of its precursors, that areconsiderably smaller than those required by the methods describedpreviously.

In particular, the effectiveness of the titanium dioxide according tothe present invention, possibly doped with metal ions belonging togroups I-VA, and the lanthanide or actinide series of the periodictable, as compared to the use of other formulations of titanium dioxide,considerably accelerates photo-oxidation of polluting agents, asillustrated in the assays described later (Examples 9, 10 and 11).

Particularly surprising is the fact that the colloidal titanium dioxideprepared according to the present invention reveals excellent adhesionto the cementitious, marble or stone material to be protected.

This was verified by subjecting specimens of cementitious and marblematerial treated with the titanium-dioxide colloidal suspension toleaching in water under stirring (Examples 7 and 8).

It is to be noted that the colloid according to the present inventionfinds in cementitious, marble or stone materials an optimal substratefor the photocatalytic action envisaged, whilst for other materials,such as glass, adhesion of the colloid is possible only via a type orthermal treatment which is particularly burdensome, at approximately 5000° C., in the presence of an organic crosslinking agent.

The activity of the colloidal preparation has moreover proved effectiveafter various cycles of fouling with coloured organic substrates andsubsequent cleaning of the specimens treated, thus demonstrating theeffect of the photocatalyst over time. Examples of implementation of thepresent invention will now be described. In these examples, thepercentage of dopant is to be understood calculated as metal-ion atomswith respect to the titanium atoms.

EXAMPLE 1 Preparation of Colloidal Titanium Dioxide

The preparation in question is based on the controlled hydrolysis of thetitanium-dioxide precursor. The typical preparation involves theaddition of titanium isopropoxide to a solution 0.1 M of HNO₃ to obtain0.565 mol. of titanium dioxide per liter.

In particular, in a 1-liter beaker containing water (750 ml) and HNO₃ ata 65% concentration (5.2 ml), titanium isopropoxide (125 ml) was addedslowly under vigorous stirring. The resulting solution was heated up to80° C. for 8-12 hours, maintaining vigorous stirring. In this way, theisopropanol that had formed was evaporated. The volume of the reactionmixture was kept at 200 ml by adding water.

EXAMPLE 2 Preparation of Colloidal Titanium Dioxide Doped With Cerium

Using cerium (Ill) nitrate hexahydrate (0.0277 g) to be dissolved in thehydrolysis water (150 ml), and titanium isopropoxide (25 ml), andfollowing the procedure described in Example 1, the product in question,containing 0.1% Ce³⁺, was obtained.

EXAMPLE 3 Preparation of Colloidal Titanium Dioxide Doped With Niobium

Using niobium (V) pentachloride (0.0223 g) to be dissolved in acetone(10 ml), and titanium isopropoxide (25 ml), and following the proceduredescribed in Example 1, the product in question, containing 0.1% Nb⁵⁺,was obtained.

EXAMPLE 4 Preparation of Colloidal Titanium Dioxide Doped With Magnesium

Using magnesium (II) chloride hexahydrate (0.0213 g) to be dissolved inthe hydrolysis water (150 ml), and titanium isopropoxide (25 ml), andfollowing the procedure described in Example 1, the product in question,containing 0.1% Mg²⁺, was obtained.

EXAMPLE 5 Preparation of Colloidal Titanium Dioxide Doped WithLanthanium

Using lanthanium (III) nitrate hexahydrate (0.037 g) to be dissolved inthe hydrolysis water (150 ml), and titanium isopropoxide (25 ml), andfollowing the procedure described in Example 1, the product in question,containing 0.1% La³⁺, was obtained.

EXAMPLE 6 Preparation of Colloidal Films on Glass Substrates

The colloid (10 ml) prepared as described in Examples 1-5 was put in ahermetically sealed glass container so as to hinder complete evaporationof the water and at the same time favour the regular growth of theparticles from an average size of 50 Å to a maximum size of 100/200 Å,and was then heated up to 200-220° C. overnight. The precipitatedcolloid was re-suspended by mechanical stirring and vacuumheat-concentrated until a concentration of 150 g/l was obtained. Next,Carbowax 2000 (300 mg) was added, and the dispersion underwent stirringfor 8 hours until total dissolution of the Carbowax was achieved and ahomogeneous suspension was obtained.

The resulting product was spread on glass substrates and set to dry in amuffle oven at 500° C. for 30 minutes. The film was characterized bymeans of diffused-reflectance spectroscopy.

Without the use of the Carbowax crosslinking agent it is impossible toget the colloid to adhere to a glass substrate.

EXAMPLE 7 Preparation of Colloidal Films on Cementitious-MortarSubstrates

The colloid (10 ml) prepared as described in Examples 1-5 was put in ahermetically sealed glass container so as to hinder complete evaporationof the water and at the same time favour the regular growth of theparticles from an average size of 50 Å to a maximum size of 100/200 Å,and was then heated up to 200-220° C. overnight.

Specimens of cementitious mortar (disks sized 25×8×2.5 cm) were preparedusing white cement Italbianco Italcementi 52.5 R.

After the preparation, the specimens were cured for 1 day in moulds inan environment at 20° C. and relative humidity (RH)>90%. After ejection,the specimens were kept for a further 7 days at 20° C. and RH≈160%.

The aqueous suspension was brush-applied on the said specimens ofcementitious mortar.

The colloid was brush-applied in small successive amounts until thetypical spectrum observed in diffused-reflectance spectroscopy wasobtained. Leaching tests ware carried out in water for 48 hours, understirring, and these tests confirmed the adhesion of the colloid. Eachtest specimen was then observed in diffracted-reflectance spectroscopy.In all, the equivalent in weight of approximately 1 g/m² was applied oneach test specimen.

EXAMPLE 8 Preparation of Colloidal Films on Marble Substrates

The colloid (10 ml) prepared as described in Examples 1-5 was put in ahermetically sealed glass container so as to hinder complete evaporationof the water and at the same time favour the regular growth of theparticles from an average size of 50 Å to a maximum size of 100/200 Å,and was then heated up to 200-220° C. overnight.

The colloidal suspension was vacuum-dried so as to obtain atitanium-dioxide powder, which was re-suspended in water (45 g/l) andbrush-applied on a specimen of white Carrara marble (dimensions, 3×3×3cm). Leaching tests were carried out in water for 48 hours, understirring, and these tests confirmed the adhesion of the colloid. Eachtest specimen was then observed in diffracted-reflectance spectroscopy.An amount of titanium dioxide corresponding in weight to approximately 1g/m² was evaluated.

EXAMPLE 9 Degradation of 4-Chlorophenol

4-chlorophenol 10 mM (3 ml) was put in an irradiation cell, in whicheach time were suspended the specimens prepared according to Example 7with the colloids prepared according to Examples 14 and containing thecatalysts according to the invention, and, as a standard specimen forcomparison, a specimen prepared in a similar way using Degussa titaniumdioxide, which, to the knowledge of the present applicant, is the mostactive photo-oxidation catalyst present on the market. The cell wasclosed under oxygen, and the degradation of the 4-chlorophenol wasmonitored by spectrophotometry, measuring the degradation times (inhours). The results are given in FIG. 1 below.

FIG. 1

Coll. TiO₂+Mg

Coll. TiO₂+Nb

Coll. TiO₂+Ce

Coil. TiO₂

Degussa TiO₂

Degradation time (hours)

As may be seen in FIG. 1, the specimens containing the dopant ions yielda time of degradation of the pollutant lower both with respect to thetest specimen containing colloidal titanium dioxide and with respect tothe standard specimen containing Degussa titanium dioxide.

EXAMPLE 10 Degradation of Naphthionic Acid

Following basically the procedure described in Example 9, but performingthe decomposition of naphthionic add 0.026 mM (3 ml) in water, anevaluation was made of the behaviour of the specimen prepared accordingto Example 7, using the titanium-dioxide colloid doped with lanthaniumaccording to classic methods and prepared as described in Example 5. Forcomparison, two non-colloidal standard specimens of Degussa titaniumdioxide were evaluated, one as such, and the other doped with lanthaniumaccording to classic methods. The results are given in FIG. 2 below.

FIG. 2

Coll. TiO₂+La

Coll. TiO₂

Degussa TiO₂+La 1%

Degussa TiO₂

Degradation time (minutes)

As may be seen in FIG. 2, the degradation time for the colloidaltitanium dioxide is lower than for the Degussa titanium dioxide, both inthe case of the specimen containing the dopant ion and in the case ofthe specimen not containing the dopant ion.

EXAMPLE 11 Degradation of 3-4-Dihydroxycinnamic Acid

Following basically the procedure described in Example 9, and using 34dihydroxycinnamic acid 0.26 mM (3 ml) in water, the following wereevaluated:

a) behaviour of the specimen prepared according to Example 7, using thetitanium-dioxide colloid doped with niobium prepared as described inExample 3;

b) behaviour of the specimen prepared according to Example 7, using thetitanium-dioxide colloid doped with magnesium prepared as described inExample 4; and

c) behaviour of the specimen of colloidal titanium dioxide preparedaccording to Example 8 on a marble test specimen.

For comparison, a standard specimen of Degussa non-colloidal titaniumdioxide was evaluated. The results are illustrated in FIG. 3.

FIG. 3

Coll. TiO₂+Mg

Coll. TiO₂+Nb

Coll. TiO₂ on marble

Coll. TiO₂ on mortar

Degussa TiO₂

Decolorizing time (minutes)

As may be seen in FIG. 3, the decolorizing times (oxidation of thepollutant) of the photocatalysts according to the present invention arein all cases shorter than for the Degussa titanium dioxide in thenon-colloidal form.

EXAMPLE 12 Degradation of Alcoholic Extract of Tobacco

Following basically the procedure described in Example 9, and usingalcoholic extract of tobacco on cement matrices, the following wereevaluated:

a) behaviour of the specimen prepared according to Example 7, using thetitanium-dioxide colloid described in Example 1;

b) behaviour of the specimen prepared according to Example 7, using thetitanium-dioxide colloid doped with cerium prepared as described inExample 2; and

c) behaviour of the specimen prepared according to Example 7, using thetitanium-dioxide colloid doped with magnesium as described in Example 4.

The alcoholic extract of tobacco was added in an amount such as toproduce a drop in transmittance from 75 to 40, read at 450 nm.

For comparison, Degussa titanium dioxide standard specimens, one at 10%and one at 0.1%, were evaluated. The results are illustrated in FIG. 4.

FIG. 4

Coll. TiO₂+Mg

Coll. TiO₂+Ce

Coll. TiO₂+Nb

Coll. TiO₂

Degussa TiO₂, 10%

Degussa TiO₂, 0.1%

Degradation time (hours)

As may be seen in FIG. 4, the times of degradation of the pollutant forthe photocatalysts according to the present invention are in all casesshorter than for the Degussa titanium dioxide in the non-colloidal form.

What is claimed is:
 1. A method for preserving the original appearanceof cementitious, stone, or marble product from the action of atmosphericagents, characterized in that the surfaces of said products are treatedwith small successive amounts of an aqueous suspension of a colorlesscolloidal preparations of titanium dioxide or one of its precursors,until the desired thickness is reached.
 2. A method according to claim1, wherein the preparations of titanium dioxide or one of its precursorscontain a metal ion chosen from the groups I-VA, and the lanthanide oractinide series of the periodic table, and mixtures thereof.
 3. A methodaccording to claim 2, wherein the preparations of titanium dioxide orone of its precursors contain a metal ion selected from the groupconsisting of lithium, beryllium, magnesium, lanthanium, cerium, andmixtures thereof.
 4. A method according to claim 3, wherein thepreparations of titanium dioxide or one of its precursors contain ionsselected from the group consisting of magnesium, cerium, and lanthanium.5. A method according to claim 2, wherein the preparations of titaniumdioxide or one of its precursors contain the metal ion in an amount offrom 0.1 to 5% (percentage expressed as metal-ion atoms with respect tothe titanium atoms).
 6. A method according to claim 5, wherein thepreparations of titanium dioxide or one of its precursors contain themetal ion in an amount of from 0.1 to 1%.
 7. A method according to claim1, wherein the titanium dioxide is prevalently in the form of anatase.8. A method according to claim 7, wherein at least 75% of titaniumdioxide is in the form of anatase.
 9. A method according to claim 1,wherein the titanium-dioxide precursor is a product able to producetitanium dioxide prevalently in the form of anatase.
 10. A methodaccording to claim 9, wherein the titanium-dioxide precursor is aproduct able to produce titanium dioxide prevalently in the form ofanatase with appropriate types of thermal treatment.
 11. A methodaccording to claim 9, wherein the titanium-dioxide precursor is chosenfrom the group comprising TiCl₄, TiOSO₄, and titanium alkoxide.
 12. Amethod according to claim 1, for the oxidation of polluting substanceschosen from the group comprising organic substances present in theenvironment as a result of motor-vehicle exhaust or industrialemissions, and inorganic compounds.
 13. A method according to claim 12,for the oxidation of nitrogen oxides (NO_(x)).
 14. A method according toclaim 1, wherein the titanium dioxide in colloidal form is preparedusing sol-gel techniques so as to obtain particles having a size ofbetween 10 and 200 Å.
 15. A method according to claim 14, wherein theparticles of titanium dioxide have a size of between 50 and 100 Å.
 16. Amethod according to claim 1, wherein the colloidal preparation isvacuum-dried so as to obtain a powder which can be re-suspended inwater, maintaining its colloidal properties.